The Boron Group: Occurrence and Physical Properties – Trends in Chemical Properties of Group 13 Elements and Their Compounds (Senior 4 Chemistry) | Ganzaa

Trends in Chemical Properties of Group 13 Elements and Their Compounds

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Senior 4 · Chemistry

Trends in Chemical Properties of Group 13 Elements and Their Compounds

4 lessons

Lesson 1 of 4

The Boron Group: Occurrence and Physical Properties

25 min

A metal that melts in your hand

Hold a piece of gallium in your palm and within a minute it turns into a silvery liquid: it melts at 29.8 °C, below body temperature. Two places above it in the same group, boron does not melt until 2075 °C, hotter than the flame of a Bunsen burner can reach. Between them sits aluminium, the metal of cooking pots, roofing sheets and drink cans. These elements all have three electrons in their outer shell, yet they behave so differently that their group is one of the most irregular in the periodic table. This unit explains why, starting with how the physical properties change from boron to thallium.

Definition

Group 13 (the boron group): the elements boron (B), aluminium (Al), gallium (Ga), indium (In), thallium (Tl) and the artificial element nihonium (Nh). Each atom has the outer configuration $ns^2\,np^1$ , three valence electrons, so the usual oxidation state in compounds is +3. Example: $\mathrm{Al}$ is $[\mathrm{Ne}]\,3s^2\,3p^1$ and forms $\mathrm{Al^{3+}}$ , $[\mathrm{Ne}]$ , in $\mathrm{Al_2O_3}$ .

Metalloid: an element with properties between those of metals and non-metals. Boron is a metalloid: it is hard and shiny like a metal, but it is a poor conductor (a semiconductor) and forms acidic oxides and covalent compounds like a non-metal.

Where they occur

None of the Group 13 elements occurs free in nature. Aluminium is the most abundant metal in the Earth's crust, about 8.2% by mass, and only oxygen and silicon are more common. It is locked up in clays, feldspars and other aluminosilicate rocks, but the only ore from which it is extracted cheaply is bauxite, a mixture of hydrated aluminium oxides formed when rocks weather in a hot, wet climate. The other elements are much rarer.

Table 8.1: Sources of the Group 13 elements

Element	Main source	Formula of the main mineral
Boron	borax and kernite, from dried-up salt lakes	$\mathrm{Na_2B_4O_7{\cdot}10H_2O}$ ; $\mathrm{Na_2B_4O_7{\cdot}4H_2O}$
Aluminium	bauxite; also cryolite (rare)	$\mathrm{Al_2O_3{\cdot}xH_2O}$ ; $\mathrm{Na_3AlF_6}$
Gallium	a by-product of purifying bauxite into alumina, and of zinc refining	no ore of its own
Indium	a by-product of refining zinc ores	no ore of its own
Thallium	a by-product of roasting sulphide ores of zinc, lead and copper	no ore of its own

How aluminium is extracted

Aluminium binds so strongly to oxygen that carbon cannot reduce aluminium oxide in a furnace, so the metal is made by electrolysis (the Hall–Héroult process). Purified alumina, $\mathrm{Al_2O_3}$ , melts at 2072 °C, far too hot for an industrial cell, so it is dissolved in molten cryolite, which lets the cell run at about 940–980 °C:

\text{cathode: } \mathrm{Al^{3+} + 3e^- \rightarrow Al(l)} \qquad \text{anode: } \mathrm{2O^{2-} \rightarrow O_2 + 4e^-}

The oxygen burns away the carbon anodes as carbon dioxide, so they must be replaced regularly. Because three moles of electrons are needed for every mole of aluminium, the process uses a great deal of electricity, which is why recycling scrap aluminium needs only about 5% of the energy used to make new metal from ore.

Physical properties down the group

Table 8.2: The Group 13 elements

Property	B	Al	Ga	In	Tl
Atomic number	5	13	31	49	81
Outer configuration	$2s^2\,2p^1$	$3s^2\,3p^1$	$3d^{10}\,4s^2\,4p^1$	$4d^{10}\,5s^2\,5p^1$	$4f^{14}\,5d^{10}\,6s^2\,6p^1$
Covalent radius / pm	85	126	124	142	144
Ionic radius of $\mathrm{M^{3+}}$ / pm	(27)	53.5	62	80	88.5
First ionisation energy / kJ mol⁻¹	800.6	577.5	578.8	558.3	589.4
Sum of first three ionisation energies / kJ mol⁻¹	6887.4	5139.0	5521.1	5083.0	5438.4
Electronegativity (Pauling)	2.04	1.61	1.81	1.78	1.62
Melting point / °C	2075	660.3	29.8	156.6	304
Density / g cm⁻³	2.37	2.70	5.91	7.31	11.8
Type of element	metalloid	metal	metal	metal	metal

The radius of $\mathrm{B^{3+}}$ is in brackets because a free $\mathrm{B^{3+}}$ ion never exists in compounds: the energy to make it is far too large (6887.4 kJ mol⁻¹).

Two big changes and two small surprises

From boron to aluminium everything changes. Boron is small and holds its electrons tightly (800.6 kJ mol⁻¹, electronegativity 2.04), so it shares electrons instead of losing them. It is a metalloid with a giant covalent structure. Aluminium has a much larger atom, a lower ionisation energy and lower electronegativity: it is a true metal.
Metallic character increases down the group overall, as atoms get bigger: aluminium to thallium are all metals, and the +3 compounds of the heavier elements are more ionic.
Surprise 1: gallium is no bigger than aluminium. Gallium has 18 more protons than aluminium, ten of them added as the 3d subshell fills. The 3d electrons are spread out and shield poorly, so the 4s and 4p electrons of gallium feel a larger effective nuclear charge. This d-block contraction keeps gallium's covalent radius (124 pm) slightly below aluminium's (126 pm), and its first ionisation energy (578.8) and electronegativity (1.81) slightly above.
Surprise 2: thallium holds its electrons more tightly than indium. Between indium and thallium the 4f and 5d subshells fill. The 4f electrons shield even worse than d electrons (the lanthanide contraction), so thallium's first ionisation energy (589.4) is higher than indium's (558.3), and its radius (144 pm) is barely larger.

Figure 8.1 shows the pattern of first ionisation energies.

Melting points: three kinds of structure

Melting points change irregularly because the structures change (Figure 8.2).

Boron forms a giant covalent network of $\mathrm{B_{12}}$ icosahedra (clusters of twelve atoms). Melting means breaking many strong covalent bonds, so it melts at 2075 °C.
Aluminium has metallic bonding with three delocalised electrons per atom and a small $\mathrm{Al^{3+}}$ ion, so it melts at 660 °C, a little above magnesium (650 °C) and far above sodium (98 °C).
Gallium has an unusual solid structure in which atoms are paired as $\mathrm{Ga_2}$ units held partly by covalent bonds, with weak forces between the pairs. Little energy is needed to collapse the solid, so it melts at 29.8 °C. Yet the liquid does not boil until about 2400 °C: on the kelvin scale its boiling point is nearly nine times its melting point, the largest ratio of any element.
Indium and thallium are soft metals with large atoms: the delocalised electrons are further from the nuclei, so metallic bonding is weaker than in aluminium.

Density rises steadily down the group, from 2.70 g cm⁻³ for aluminium to 11.8 g cm⁻³ for thallium, because the mass of the atom increases much faster than its volume.

Pieces of very pure thallium sealed in a glass ampoule under argon. Thallium is a soft, heavy metal (11.8 g cm⁻³) that tarnishes in air, and it and its compounds are highly toxic, so it is never handled in school laboratories. Photo: W. Oelen, CC BY-SA 3.0, via Wikimedia Commons

Worked example 1: reading successive ionisation energies

The first five ionisation energies of boron are 800.6, 2427.1, 3659.7, 25 025.8 and 32 826.7 kJ mol⁻¹. Show how these values place boron in Group 13, and explain why boron does not form $\mathrm{B^{3+}}$ ions in its compounds.

Solution.

Ratios of successive values: $\frac{2427.1}{800.6} = 3.0$ , $\frac{3659.7}{2427.1} = 1.5$ , but $\frac{25\,025.8}{3659.7} = 6.8$ . The big jump comes after the third electron, so the fourth electron is in an inner shell (1s). Boron has three outer electrons: Group 13.
Making $\mathrm{B^{3+}}$ costs $800.6 + 2427.1 + 3659.7 = 6887.4\ \mathrm{kJ\,mol^{-1}}$ , about 1750 kJ mol⁻¹ more than aluminium needs (5139.0). The tiny $\mathrm{B^{3+}}$ ion would also pull so hard on any neighbouring electrons that the bond would be covalent anyway. So boron shares its three electrons in covalent bonds, as in $\mathrm{BF_3}$ and $\mathrm{B_2O_3}$ .

Worked example 2: comparing aluminium and gallium

(a) Calculate the energy needed to turn 2.70 g of gaseous aluminium atoms into gaseous $\mathrm{Al^{3+}}$ ions. $A_r(\mathrm{Al}) = 26.98$ . (b) Gallium has more electron shells than aluminium, yet the same change for one mole of gallium needs more energy. Explain.

Solution. (a) For one mole: $\mathrm{Al(g) \rightarrow Al^{3+}(g) + 3e^-} \qquad \Delta H = 577.5 + 1816.7 + 2744.8 = 5139.0\ \mathrm{kJ\,mol^{-1}}$ $n(\mathrm{Al}) = \frac{2.70\ \mathrm{g}}{26.98\ \mathrm{g\,mol^{-1}}} = 0.1001\ \mathrm{mol} \qquad E = 0.1001 \times 5139.0 = 514\ \mathrm{kJ}$ (b) Gallium needs $5521.1\ \mathrm{kJ\,mol^{-1}}$ , 382 kJ mol⁻¹ more. Its nucleus has 18 more protons than aluminium's, and ten of the extra electrons are 3d electrons, which shield the outer 4s and 4p electrons poorly. The outer electrons therefore feel a larger effective nuclear charge, which more than cancels the effect of the extra shell.

Warning

Common mistakes. (1) Expecting every property to change smoothly down Group 13 as it does in Groups 1 and 2: the filled 3d and 4f subshells make gallium and thallium "smaller" than expected. (2) Calling boron a metal because it is shiny: it is a metalloid with a covalent network. (3) Writing $\mathrm{B^{3+}}$ in a formula such as " $\mathrm{B^{3+}(F^-)_3}$ ": boron compounds are covalent. (4) Explaining gallium's low melting point by "weak metallic bonding because the atom is big": its atoms are no bigger than aluminium's; the cause is its unusual paired structure.

Safety. Thallium and its compounds are highly toxic by swallowing, breathing in dust or contact with skin, and are not used in schools. Gallium is low in toxicity but stains skin and attacks aluminium, making it brittle, so it is handled with gloves and kept away from aluminium objects. Wear eye protection when handling any metal sample.

Tip

Group 13 in one line: three outer electrons, a covalent metalloid at the top and soft metals below, with "d and f shielding" spoiling the smooth trends at gallium and thallium. When a Group 13 trend looks wrong, look at what filled up just before that element.

Professor Dave Explains: Periodic Table Part 4, the Boron Group (10 min). Watch for how boron differs from the metals below it, why aluminium is extracted by electrolysis, and the uses of gallium and indium in electronics.

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Practice

Write the full electron configurations of aluminium and of gallium, and give the outer configuration that all Group 13 atoms share.
Name the ore from which aluminium is extracted, and explain why the alumina is dissolved in molten cryolite before electrolysis.
Explain why boron melts at 2075 °C but aluminium melts at 660 °C.
The first ionisation energy of gallium (578.8 kJ mol⁻¹) is slightly higher than that of aluminium (577.5 kJ mol⁻¹). Explain why.
Calculate the energy needed to turn 1.00 g of gaseous indium atoms into gaseous $\mathrm{In^{3+}}$ ions. Use Table 8.2; $A_r(\mathrm{In}) = 114.82$ .
An element X has successive ionisation energies of 578, 1817, 2745, 11 577 and 14 842 kJ mol⁻¹. Which group is X in? Explain your answer and suggest what X is.
Thallium forms many stable compounds of the $\mathrm{Tl^{+}}$ ion, such as TlCl, while aluminium forms only $\mathrm{Al^{3+}}$ . Use the second and third ionisation energies in Table 8.2 and the ionic radii to suggest why.

Show answersHide answers

Al: $1s^2\,2s^2\,2p^6\,3s^2\,3p^1$ ; Ga: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^{10}\,4s^2\,4p^1$ ; outer configuration $ns^2\,np^1$ .
Bauxite. Alumina melts at 2072 °C; dissolved in cryolite the mixture is molten at about 940–980 °C, which saves energy and makes the cell practical.
Boron has a giant covalent network ( $\mathrm{B_{12}}$ icosahedra): melting breaks many strong covalent bonds. Aluminium is held by metallic bonding, which is strong but needs far less energy to loosen into a liquid.
The ten 3d electrons added before gallium shield the outer electrons poorly, so gallium's 4p electron feels a larger effective nuclear charge; this outweighs its extra shell.
$I E_{1} + I E_{2} + I E_{3} = 5083.0$ kJ mol⁻¹; $n = \frac{1.00}{114.82} = 0.008709$ mol; energy $= 0.008709 \times 5083.0 = 44.3$ kJ.
Group 13: the large jump comes between the third (2745) and fourth (11 577) values, so X has three outer electrons. The values match aluminium.
For Tl, $I E_{2} + I E_{3} = 1971 + 2878 = 4849$ kJ mol⁻¹, more than for Al ( $1816.7 + 2744.8 = 4561.5$ ), so the two 6s electrons are hard to remove. $\mathrm{Tl^{3+}}$ (88.5 pm) is also much larger than $\mathrm{Al^{3+}}$ (53.5 pm), so less lattice or hydration energy is released to repay the cost. Losing only the 6p electron, to give $\mathrm{Tl^{+}}$ , is the better bargain.

Quick check

Gallium has one more electron shell than aluminium, yet its covalent radius (124 pm) is slightly smaller. What is the best explanation?

Key points

Group 13 atoms are $ns^2\,np^1$ and usually show the +3 oxidation state; thallium also forms stable +1 compounds.
Boron is a metalloid with a giant covalent network; aluminium to thallium are metals. Aluminium (8.2% of the crust) is extracted from bauxite by electrolysis of alumina in molten cryolite.
Radius and metallic character increase down the group, but poor shielding by 3d and 4f electrons makes gallium no bigger than aluminium and gives gallium and thallium higher ionisation energies than expected.
Melting points follow structure: boron (covalent network) 2075 °C, aluminium 660 °C, gallium (paired atoms) only 29.8 °C.

Sources and credits

Image: Gallium crystals.jpg by en:user:foobar, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Image: Boron R105.jpg by James L. Marshall, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Image: Thallium pieces in ampoule.jpg by W. Oelen, via Wikimedia Commons, licensed under CC BY-SA 3.0.
Data: ionisation energies from Wikipedia's ionisation energies data page (CRC Handbook values); covalent radii from the atomic radii data page; Shannon's six-coordinate ionic radii from the Ionic radius article; melting points, densities and electronegativities from the PubChem Periodic Table, National Library of Medicine.
Crustal abundance of aluminium: Wikipedia, Abundance of elements in Earth's crust; cell temperature: Hall–Héroult process; recycling energy: Aluminium recycling; gallium's liquid range: Gallium; all Wikipedia, CC BY-SA 4.0, adapted by Ganzaa.
Video: Periodic Table Part 4: Boron Group (B, Al, Ga, In, Tl, Nh), Professor Dave Explains, embedded from YouTube.
The text, charts, tables, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A metal that melts in your hand

2Where they occur

3How aluminium is extracted

4Physical properties down the group

5Two big changes and two small surprises

6Melting points: three kinds of structure